ESD damage is a well known phenomenon which can occur during the fabrication of semiconductor devices such as metal-oxide semiconductor structures. In particular, ESD damage can result in gate insulating layer breakdown, large shifts in threshold voltages, and large leakage currents between transistor electrodes.
One important kind of consumer and commercial LCD is the thin film transistor liquid crystal display (TFT-LCD). During a typical TFT-LCD fabricating process, static electricity may be generated on the TFT-LCD. If a large amount of electrical charge builds up at any one location on the TFT-LCD, the built up electrical charge is liable to discharge and thereby damage or destroy internal components of the TFT-LCD such as thin film transistors. To avoid the damaging effects of static electricity buildup and discharge during and after forming of the TFT-LCD array, a plurality of ESD protection circuits are employed in a typical TFT-LCD.
FIG. 4 is a schematic, abbreviated diagram of a layout of a conventional TFT LCD having a plurality of ESD protection circuits. The TFT-LCD 100 includes a first substrate (not shown), a second substrate (not shown), and a liquid crystal layer (not shown) sandwiched between the two substrates. The first substrate includes a plurality of gate lines 10 that are parallel to each other and that each extend along a first direction, a plurality of data lines 11 that are parallel to each other and that each extend along a second direction orthogonal to the first direction, a display region 12 (denoted by a broken line), common electrodes 13 formed at a periphery of the TFT-LCD 100, and a plurality of ESD protection circuits 14. The ESD protection circuits 14 are electrically connected between the common electrodes 13 and each of the gate lines 10 as well as between the common electrodes 13 and each of the date lines 11, respectively.
Also referring to FIG. 5, each ESD protection circuit 14 includes a first transistor 15, a second transistor 16, a third transistor 17, a first terminal 18, and a second terminal 19. Each transistor 15, 16 and 17 includes a source electrode ‘s’, a drain electrode ‘d’, and a gate electrode ‘g’. The drain electrode ‘d’ and gate electrode ‘g’ of the first transistor 15 are connected to each other. The drain electrode ‘d’ and gate electrode ‘g’ of the second transistor 16 are connected to each other. The gate electrode ‘g’ of the third transistor 17 and the source electrodes ‘s’ of the first and second transistors 15 and 16 are connected to each other. The drain electrode ‘d’ of the first transistor 15 and the source electrode ‘s’ of the third transistor 17 are connected together to define the first terminal 18. The drain electrodes ‘d’ of the second and third transistors 16 and 17 are connected together to define the second terminal 19. The first terminal 18 is connected to one of the gate lines 10 or one of the data lines 11, and the second terminal 19 is connected to one of the common electrodes 13. The first and second transistors 15 and 16 are serially connected between the first and second terminals 18 and 19.
Generally, operation of the ESD protection circuit 14 is as follows. When an electric potential difference generated by an ESD is greater than that of the serially connected first and second transistors 15 and 16, the third transistor 17 is turned on and a charge on the first terminal 18 flows to the common electrode 13 via the third transistor 17. Thus the ESD protection circuit 14 can prevent static electricity, generated in the process of fabricating the corresponding gate line 10 or data line 11 of the TFT-LCD 100, from destroying other internal circuits (not shown) of the TFT LCD 100.
However, in general, each of the gate lines 10 and data lines 11 has an operation voltage of 5V, which is greater than the threshold voltage of the corresponding ESD protection circuit 14. Therefore the third transistor 17 is liable to generate leakage current when the ESD protection circuit 14 in an off state. The sum of leakage currents of all the ESD protection circuits 14 causes the TFT-LCD 100 to have unduly high power consumption. In addition, the leakage currents of the ESD protection circuits 14 are liable to cause a voltage of each of the common electrodes 13 to waver. This in turn may lead to flickering occurring on a display screen of the TFT-LCD 100.
Accordingly, what is needed is an ESD protection circuit for a TFT-LCD that can overcome the above-described deficiencies.